Effect of Heat Treatment on Dielectric Strength of Polyethylene Terephthalate under Compressive Stress

Measurements of dc dielectric strength of polyethylene terephthalate (PET) film at room temperature are reported when it is heat set in silicone oil over a range of temperature between 60 to 200°C for durations of 1 and 60 min in order to control the degree of crystallinity of the sample. It is found that the dc dielectric strength under no mechanical stress increases with an increase in the degree of crystallinity, but the maximum dielectric strength under compressive stress at first increases with the degree of crystallinity, then reaches a maximum value, and decreases thereafter. For the samples heat treated near the glass transition temperature, which is about 70°C, the characteristics of dielectric strength under compressive stress are also varied significantly with heat-treated temperature and time although the crystallinity is constant.